my FreeNAS. However, 16GB Ultra Fit drives are *really* 15GB. My source disks: da0: <Staples 1.22> Direct Access SCSI SPC-3 Removable Deviceda0: 15267 MB (31266816 sectors of 512 bytes) da1: <SanDisk Cruzer Fit 1.27> Direct Access SCSI SPC-4 Removable Deviceda1: 15267 MB ( 31266816 512 bytes sectors) However, all new drives are about 600 MB smaller: da2: <SanDisk Ultra Fit 1.00> SPC-4 SCSIda2 Direct Access Removable Device: 14664 MB (30031872 512-byte sectors). Small enough that you can't mirror any of the older drives. It's ok if you don't need the full 16GB.
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