External USB drive - primary GPT table is corrupt or invalid

[Gianni]

I've created a single slice and partition on an external USB drive  
using sysinstall which gives the following devices:
/dev/da0
/dev/da0s1
/dev/da0s1c
/dev/da0s1d

I can mount da0s1d and read+write data just fine but I get the  
messages below when I initially attach the drive; can someone explain  
what "the primary GPT table is corrupt or invalid" means and if it's  
something I should be concerned about.

umass0: <JMicron JM20336 SATA, USB Combo, class 0/0, rev 2.00/1.00,  
addr 4> on uhub4
da0 at umass-sim0 bus 0 target 0 lun 0
da0: <WDC WD32 WD-WCARW278 58.0> Fixed Direct Access SCSI-2 device
da0: 40.000MB/s transfers
da0: 305245MB (625142448 512 byte sectors: 255H 63S/T 38913C)
GEOM: da0: the primary GPT table is corrupt or invalid.
GEOM: da0: using the secondary instead -- recovery strongly advised.
GEOM_LABEL: Label for provider da0p1 is msdosfs/EFI.

Thanks
Gianni