In the external disk array market, it’s surprising how little changes in terms of the top five vendors’ market shares quarter-to-quarter or even year-to-year. The leading vendors almost always occupy the same rungs on the ladder. However, based on IDC’s Q1 2010 stats, market shares appear to be shifting.

Of course, EMC held on to its #1 spot in the first quarter with a 24.6% market share on revenue of $1.22 billion. But the rest of the race is tightening.

For example, for the first time NetApp pulled into a virtual dead heat with IBM for the #2 spot in the external disk systems market. IBM had an 11.7% market share, while NetApp had an 11.1% share. Statistically, IDC considers that to be a tie. Compared to Q1 2009, IBM slipped 0.5% in terms of market share while NetApp gained 2.3 points, leapfrogging both Dell and HP, which were in the #3 and #4 spots in the previous quarter.

If NetApp’s ascendancy continues at this clip, the company could eclipse Big Blue. And if NetApp’s first quarter and year-end earnings are any indication (see “NetApp wows Wall Street, doubles quarterly profits”), I think they will.

Rounding out the top five, HP slipped a bit in the first quarter, to a 10.2% market share, followed closely by Dell with a 10.1% share. Dead heat. And I predict that Dell will overtake HP in the next quarter because the revenue differential between the two was only $6 million in the first quarter ($506 million vs. $500 million).

Hitachi Data Systems and Oracle/Sun failed to crack the top five.

NetApp also gained market share in the NAS arena over the last quarter with a 26.9% share (vs. 20.2% in the last quarter), coming in second to EMC’s 45.1% market share (down from a 50.5% chunk in the last quarter).

Overall, the NAS market grew a whopping 44.6% year-over-year.

Equally impressive, the iSCSI SAN market posted 45.7% revenue growth in Q1 2010 vs. Q1 2009. Dell led the iSCSI market with a 36.9% revenue share, followed by NetApp with a 14.4% slice.