I have a marantz Pmd661 and the problem that I have with it is that when I try to insert the memory card, it bounces back and I have to push and close close the door in order for it to stay inside the machine, but still it gives Unformat error message. What could be the problem?

For the best answer contact Korg customer service. Here's the link-
http://www.korg.com/us/support/contact

You have a memory problem. If you are using a flash card for memory or a CD drive or perhaps you have configured an external drive, you need to free up some space. Try deleting tracks or songs you don't need anymore.

Each song is independent on a CompactFlash
without affecting other songs. You can give a desired song name for managing recorded songs
(see pages 23 and 91).
Recorded track data is stored in a song as mono WAV files.
The MR-8mkII can convert the two WAV files recorded on tracks 7 and 8 (L and R) to a stereo
WAV file (see page 115).
You can export the converted stereo WAV file to your USB-connected personal computer, and
use the file by a music software application or make an original audio CD on the computer using
a CD burn software application. You can also burn an audio CD by using an external CD-R/RW
drive connected via the [USB HOST] port of the MR-8mkII (see page 115 and 123)
Fostex MR 8MKII Owner Manual Page 73 Fostex MR 8MKII Owner Manual Exporting Data To Personal Computer...

Are you using the auto delete feature? Get rid of data you don't need here: http://www.manualslib.com/manual/53326/Fostex-Mr-8mkii.html?page=135#manual

Yes you can simply replace 4gb card with 8gb, 16gb, ect...
the device doesn't know the difference, it just gives you more storage capacity... hope this helps?

This was in the manual:
Card too busy. Repeated actions of recording, writing, and deleting will gradually decrease the amount of contiguous free space on the memory card, causing files to be saved in a fragmented state during recording. This message will appear when access to the memory card has slowed down due to such problems. Back up your important files, format the memory card, and then restore your files back onto the card.

If you let the input "float" with no connection it very well may buzz. The input needs to be tied down with a driving source.

It’s necessary to bounce all of your tracks to a stereo pair of tracks before you can export your song to a computer as a WAV or MP3 file for burning onto a CD.
1. In normal mode, push EXIT and UTILITY until you see BOUNCE on the display.
Note: The upper right corner of the display shows the track you’re bouncing to. By default, “12V2”—Tracks 1 and 2, Virtual Track 2—is selected.
2. Press STOP and REWIND together to go back to the beginning of the song.
3. Press RECORD, and then press PLAY.
4.When the song’s finished playing, press STOP.
i read some stuff online and this should help.

Refer to your BR manual for procedure to format the card. Formatting the card in a PC just doesn't cut it.

Look in your index in the manual for "formatting" or "card"...etc.

You need to verify that your card is confirmed to work in the MR8 at this site:

http://www.fostexinternational.com/docs/support/MR8_Operation_CF_20071213.pdf

It is OBVIOUS that your card is not acceptable because it is larger than 2GB.

http://safemanuals.com/user-guide-instructions-owner-manual/AKAI/MPC2500-_E This is a free download. Try pages 14 and 15 also 23 also44 also page 66 also 83 also 90(purge samples) 103 (memory cards) 11 formatting page 108 deleting data in memory before loading, I'd suggest page 112 (save the contents by backing up to computer) then wiping the memory card. I don't have one of these units and I see the hard drive is an option which I don't know if you have. Read the manual from beginning top end and always save your work before deleting anything. Hope this helps.

Remove card, reincert card and initialize again. card may also be fragmented, use card optimizer in menu.