is the laptop not seeing the external usb harddisk? Insufficient power could be the cause of the 'disappearing' act. Try removing the external USB optical drive and leaving only the ext harddisk plugged in.
While prepping the laptop for a larger hard drive, I found that the current drive (Seagate 100 gig SATA I) was not visible in Disk Management. The CD ROM device was there. In further troubleshooting I find that no USB external hard drive partition will show in disk management but USB CD ROM devices will. The hard drive shows in device manager and explorer just fine. With the exception of an an occasion shut down issue, the laptop appears to be working normally. In attempting to run a few diagnostics, I find that the Lenovo "Disk Doctor" shows no problems with the hard drive but, I cannot schedule chkdsk /f. It reboots normally without running the program. sfc /scannow runs and shows no errors. I also noted that no system restore points were being made though system restore was enabled. I disabled/re-enabled system restore and it seems to be ok now. I've also re-enabled all default services but no luck. I cannot run defrag as well. I've not yet gone the repair installation as what we have is the recovery media burned when we first got the laptop. I'm not wanting to risk making matters worse at the moment. I've made an Acronis image of the drive in its current condition in case this happens.
As I said, the laptop seems to run ok. I don't want to clone the old hard drive with problems on it, however. Any thoughts as to why no chkdks /f or the disk showing either of it's partitions in disk management?