This exact issue is happening to me. New drive (Samsung HD203WI) in an external USB case (Eagle Consus). Fresh install of Lucid x64 on a re-purposed Dell desktop.

Experiencing intermittent USB drive disconnects during sustained transfer of files from another local machine via rsync/ssh.

Dell Optiplex GX620 3.5GB RAM
Pentium D Dual-Core
Ubuntu Server Lucid 10.04 x64
2.6.32-25-server #44-Ubuntu SMP x86_64

From kern.log:
============

Oct 17 22:13:34 redsquare2 kernel: [  512.950159] usb 1-7: reset high speed USB device using ehci_hcd and address 3
Oct 17 22:13:34 redsquare2 kernel: [  513.020311] usb 1-7: USB disconnect, address 3
Oct 17 22:13:34 redsquare2 kernel: [  513.022053] sd 4:0:0:0: [sdb] Unhandled error code
Oct 17 22:13:34 redsquare2 kernel: [  513.022060] sd 4:0:0:0: [sdb] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK
Oct 17 22:13:34 redsquare2 kernel: [  513.022067] sd 4:0:0:0: [sdb] CDB: Write(10): 2a 00 46 b3 be ea 00 00 f0 00
Oct 17 22:13:34 redsquare2 kernel: [  513.022080] end_request: I/O error, dev sdb, sector 1186184938
Oct 17 22:13:34 redsquare2 kernel: [  513.022088] Buffer I/O error on device sdb5, logical block 105807290
Oct 17 22:13:34 redsquare2 kernel: [  513.022091] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022097] Buffer I/O error on device sdb5, logical block 105807291
Oct 17 22:13:34 redsquare2 kernel: [  513.022100] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022103] Buffer I/O error on device sdb5, logical block 105807292
Oct 17 22:13:34 redsquare2 kernel: [  513.022106] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022110] Buffer I/O error on device sdb5, logical block 105807293
Oct 17 22:13:34 redsquare2 kernel: [  513.022113] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022116] Buffer I/O error on device sdb5, logical block 105807294
Oct 17 22:13:34 redsquare2 kernel: [  513.022119] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022122] Buffer I/O error on device sdb5, logical block 105807295
Oct 17 22:13:34 redsquare2 kernel: [  513.022125] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022129] Buffer I/O error on device sdb5, logical block 105807296
Oct 17 22:13:34 redsquare2 kernel: [  513.022131] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022135] Buffer I/O error on device sdb5, logical block 105807297
Oct 17 22:13:34 redsquare2 kernel: [  513.022138] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.022141] Buffer I/O error on device sdb5, logical block 105807298
Oct 17 22:13:34 redsquare2 kernel: [  513.022144] lost page write due to I/O error on sdb5
Oct 17 22:13:34 redsquare2 kernel: [  513.025257] sd 4:0:0:0: [sdb] Unhandled error code
Oct 17 22:13:34 redsquare2 kernel: [  513.025262] sd 4:0:0:0: [sdb] Result: hostbyte=DID_NO_CONNECT driverbyte=DRIVER_OK
Oct 17 22:13:34 redsquare2 kernel: [  513.025267] sd 4:0:0:0: [sdb] CDB: Write(10): 2a 00 46 b3 bf da 00 00 f0 00
Oct 17 22:13:34 redsquare2 kernel: [  513.025281] end_request: I/O error, dev sdb, sector 1186185178
Oct 17 22:13:35 redsquare2 kernel: [  513.078583] JBD: Detected IO errors while flushing file data on sdb5
Oct 17 22:13:35 redsquare2 kernel: [  513.096943] Aborting journal on device sdb5.
Oct 17 22:13:35 redsquare2 kernel: [  513.112412] ext3_abort called.
Oct 17 22:13:35 redsquare2 kernel: [  513.112421] EXT3-fs error (device sdb5): ext3_journal_start_sb: Detected aborted journal
Oct 17 22:13:35 redsquare2 kernel: [  513.112425] Remounting filesystem read-only
Oct 17 22:13:35 redsquare2 kernel: [  513.122593] JBD: Detected IO errors while flushing file data on sdb5
Oct 17 22:13:35 redsquare2 kernel: [  513.205534] __journal_remove_journal_head: freeing b_committed_data
Oct 17 22:13:35 redsquare2 kernel: [  513.234243] EXT3-fs error (device sdb5): ext3_find_entry: reading directory #2 offset 0
Oct 17 22:13:35 redsquare2 kernel: [  513.234399] EXT3-fs error (device sdb5): ext3_find_entry: reading directory #2 offset 0
Oct 17 22:13:35 redsquare2 kernel: [  513.235349] EXT3-fs error (device sdb5): ext3_find_entry: reading directory #2 offset 0
Oct 17 22:13:35 redsquare2 kernel: [  513.475982] ext3_abort called.
Oct 17 22:13:35 redsquare2 kernel: [  513.475990] EXT3-fs error (device sdb5): ext3_put_super: Couldn't clean up the journal
Oct 17 22:13:35 redsquare2 kernel: [  513.476208] JBD2: I/O error detected when updating journal superblock for sdb3-8.
Oct 17 22:13:35 redsquare2 kernel: [  513.476391] EXT4-fs (sdb3): previous I/O error to superblock detected
Oct 17 22:13:35 redsquare2 kernel: [  513.478929] JBD2: I/O error detected when updating journal superblock for sdb2-8.
Oct 17 22:13:35 redsquare2 kernel: [  513.479127] EXT4-fs (sdb2): previous I/O error to superblock detected
Oct 17 22:13:35 redsquare2 kernel: [  513.520051] usb 1-7: new high speed USB device using ehci_hcd and address 4
Oct 17 22:13:35 redsquare2 kernel: [  513.671320] usb 1-7: device descriptor read/64, error -71
Oct 17 22:13:35 redsquare2 kernel: [  513.851516] hub 1-0:1.0: unable to enumerate USB device on port 7