Garbh Choire on Bheinn a'Bhuird is a remote place to visit any time of the year, but in winter/spring, it feels even more so. That said, it's quite feasible if conditions allow, to use a bike to take a lot of the sting out of the return trip, and it's possible to get here for a day and do more than one run into the corrie.
Getting there.
Although not essential, use of a bike from either direction will make both ends of the day easier.
Tomintoul / Corgarff approach: This involves a fair bit of ascent and descent on the bike (both directions) before you set off on foot from the footbridge on the River Avon at NJ126059, particularly if you park at Corgarff. The path from the footbridge up Slochd Mór into Garbh Choire is indistinct, and easily lost.
Glen Quoich approach: You should be able to cycle to the foot of the An Diollaid spur, although if the river crossing is difficult you may have to leave the bikes there. You can either walk or skin up An Diollaid (usually snow on the east side of the ridge), arriving on the plateau at the head of Coire an Dubh Lochan. This approach may be preferable if you don't like cycling uphill too much with skis on your back, but it does mean that you will expend effort to get back onto the plateau for an easy run down (which may be welcome!).
Gleann an t-Slugain approach: The landrover track from Keiloch is cyclable all the way up to the head of the glen, except for a couple of short steep sections. It is worth pushing the bike up these if there is minimal snow cover. Once over the top, there is a new path up the glen. It is also cyclable for another 2.5km, if you don't mind some drainage ditches. This approach means that you can finish down a run in one of the corries, and the descent on the mountain bike is good as well!
Return to the plateau
The large open slope to the West of Mitre ridge is the easiest route back to the plateau (at about NJ105015). Make sure you don't cut up too soon behind Mitre Ridge, as there is another buttress below the plateau. This slope is in itself is a great descent, particularly if you cut in below Mitre Ridge. The 'Sneck' is the col between Bheinn a'Bhuird and Ben Avon. If heading south, you may have to walk a fair bit of it, but it offers the least ascent back out of the corrie.
Relevant Weather Forecasts
