Drive A: (and B: ) are hardcoded/hardwired build-in left-over relics of non-mobile devices, but still valid when not de-activated. So searches over all drives will also include A:, B:, C:, etc. I do not think calibre should solve this.
For WinXP, there existed a tweak utility to let windows "know" not to use drive A, B by unchecking some check-box. Did a quick search for something similar on my Win7, but no luck (have no A/B floppy drive anyway).
Further, as windows allows renaming drive indications, drive A: might actually map to some exsisting external USB-drive. Another reason not to solve this in calibre.
Just my 2cts.