How to to mount other drives in FreeDOS

What file systems are supported?

FAT32 is fully supported and is the preferred format for the boot drive.

Depending on the BIOS used, up to four LBA (Logical block addressing) hard disks up to 128 GB, or 2 TB, in size are supported.[38] There has been little testing with large disks, and some BIOSes support LBA but produce errors on disks larger than 32 GB; a driver such as OnTrack or EZ-Drive resolves this problem.

FreeDOS can also be used with a driver named LFNDOS to enable support for Windows 95-style long file names, but most old programs before Win95 do not support LFNs even with driver loaded unless they have been recompiled.

There is no planned support for NTFS, ext2 or exFAT, but there are several external third-party drivers available for that purpose. To access ext2fs, LTOOLS (counterpart to Mtools) can sometimes be used to copy data to and from ext2fs drives.

Source FreeDOS

CDs and USBs is what I'm really interested in

Freedos supports CR ROM drives using xcdrom.sys

XCDROM.SYS is an UDMA and non-UDMA CD-ROM driver for DOS. XCDROM.SYS has to be loaded in CONFIG.SYS / FDCONFIG.SYS. When FreeDOS is already running, you can load XCDROM.SYS later with DEVLOAD

Source Command: xcdrom.sys

There are DOS USB drivers available, for example at DOS USB Drivers.

I've no idea whether they work with FreeDOS.