how do I scan a removable disk?

Thanks, managed to work it out myself

Open Windows File Explorer, chose the drive that contains the disk, right click it and chose "scan with Webroot" from the context menu. 

@ wrote:
Thanks, managed to work it out myself

Welcome to the Community @, you just beat me to it, I was going to tell you how to do it.

Thanks for that info. Still using Windows XP it only want to scan using Windows security Essentials on option to choose Web Root. Have worked out how to do a :Custom Scan" which solves my problem.

Just like most things, there is also more than one way to get the job done.  You can also do a Customer Scan
 

1. Open WSA
2. Click the Gear Tool next to PC Security
3. Click the Scan & Shields tab
4. Click the Customer Scan button
5. Click the Add File/Folder button
6. Select the DRIVE that you want to scan.
7. Click Add
8. Repeat steps 5 through 7.  You can add as many drives, folders and individual files that you want.
9. Click Start Scan

 
Not as one click and go obviously as the Windows Explorer, right click and scan, but it can be very handy when you need to set up a scan of multiple external drives or multiple selected files.
 
Keep your eyes open and don't blink if scanning an SD card,,, my 8 Gb SD card scans in about 2 seconds.  : )
 
I hope this helps,