Airport Extreme Constantly Losing Backup Disk

[YatBob]

I have an Airpot Extreme (latest model) with a WD 2tb external HD connected which I use for backing up my iMac, every couple of days when Time Machine goes to do a backup I get a message "backup disk could not be found", I have to then restart the Airport Extreme and all works well for a couple of days, then repeats the cycle

Any suggestions

 

[Altemose]

Connect the disk to a Mac via USB and verify the disk in Disk Utility.

 

[satcomer]

Plus this tells me your cheap external disk is going into 'sleep mode'!