The surface of a floppy disk is very fragile, and are easily damaged by constant use in machines that other people use. You never know if someone has contaminated the drive you just put your disk into. 12 Microinches would be written as 0.000012 inches, and 30 Microinches as 0.000030 inches. Be very careful with your diskettes. Hard disks are generally well protected from contaminants, but can still become contaminated if the drive case is exposed to extreme temperature variations.
Seals do leak once in a while.
While Hard Disk heads are generally protected from the elements, your floppy disks are not as fortunate. If you put a diskette in a drive, and there is any chance that your disk has been contaminated, you can literally scrape your data right off the diskette. If you think you can safely use a disk, be absolutely sure that your disk has been as comfortable as you. Keep it Clean and dry, and use of a floppy disk cleaning kit will lessen the possibility of a head crash that can damage your floppy disk. We have seen many of you in our shop, needing data recovery for term papers and dissertations, at the last minute, because you have not been practicing 'Safe Diskette'.