Run WinUAE as administrator, load your favourite Workbench config, go to Hard Drives page, click on Add Hard Drive, select your SCSI HDD, check read/write and click on Add, then click on Start.
If Workbench does not recognise the HDD, then it's probably not compatible to the Commodore RDB / RDSK standard. You could still mount the partitions if you create a mount list for each partition.
