Thanks for the guidance on SDFormatter. Unfortunately, it's a windows or mac-only thing, and I'm running Ubuntu.
Some extensive research on the 'net about SD cards seems to indicate they don't really need much in the way of care and feeding on formatting, except that skipping the first 4M of the card will result in higher write performance.
Ubuntu recognizes the card (via USB reader) as:
[55527.927703] scsi 6:0:0:0: Direct-Access Mass Storage Device 1.00 PQ: 0 ANSI: 0 CCS
[55527.927937] sd 6:0:0:0: Attached scsi generic sg1 type 0
[55528.193547] sd 6:0:0:0: [sdb] 15644672 512-byte logical blocks: (8.01 GB/7.45 GiB)
[55528.193761] sd 6:0:0:0: [sdb] Write Protect is off
[55528.193763] sd 6:0:0:0: [sdb] Mode Sense: 03 00 00 00
[55528.193916] sd 6:0:0:0: [sdb] No Caching mode page found
[55528.193917] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[55528.194961] sd 6:0:0:0: [sdb] No Caching mode page found
[55528.194964] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[55528.196011] sdb: sdb1
[55528.196864] sd 6:0:0:0: [sdb] No Caching mode page found
[55528.196866] sd 6:0:0:0: [sdb] Assuming drive cache: write through
[55528.196878] sd 6:0:0:0: [sdb] Attached SCSI removable disk
fdisk reports that indeed, the first partition starts 4MB into the device:
Disk /dev/sdb: 8010 MB, 8010072064 bytes
214 heads, 8 sectors/track, 9138 cylinders, total 15644672 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x00000000
Device Boot Start End Blocks Id System
/dev/sdb1 8192 15644671 7818240 b W95 FAT32
...So I'm not sure what any special formatter is going to do for me.
I'm now executing a full physical read of the device in Ubuntu (with dd) to verify I really have 8GB here, although it seems unlikely this would be counterfeit (being Lexar brand purchased at a brick-and-mortar Office Depot local to me here in Raleigh, NC).
Also note, I have been unable to reproduce this problem with comparable logic written in Perl on my linux machine, with the same card, which strongly points to something with the arduino SD (or maybe seek) lib implementation.